The present invention relates to a semiconductor gas sensor, and particularly to a combined semiconductor gas sensor constituted by two different gas sensitive materials, which has high sensitivity and preferable selectivity, thermo-stability and anti-humidity capability.
Recently, semiconductor gas sensors have become one of the most rapidly developed and widely used gas sensors for their advantages of relatively high sensitivity, uncomplicated manufacturing techniques, convenient and flexible usage, etc. More and more types of semiconductor gas sensors have also been developed, the kinds of gases that can be detected have been increasing rapidly, from the detection of flammable and explosive gas to the detection of poisonous gases, fragrance, stink, as well as freshness. Some of the gases used in producing of semiconductors and integrated circuits are of severe poison, the density to be detected is in the range of 1-100 ppm, and sometimes even in the magnitude of ppb's. In view of poisoning preventing and personal safety, it is also required to be extended to the detection of low density ones in many occasions. For example, for preventing slow poisoning, the allowable average density of H.sub.2 S for eight-hour working is 15 ppm, and 150 ppm for CO; for preventing acute poisoning, the allowable density for fifteen-minute exposure to H.sub.2 S is 100 ppm, and to CO is 400 ppm. The existing gas sensors can hardly satisfy this measuring requirement.
As for methods for improving the sensitivity of semiconductor gas sensors, the recently adopted methods include:
1. adding catalyst to improve the activity of gas sensitive materials; PA1 2. super-parculate of the material; PA1 3. searching for new materials with desirable sensitivity. PA1 (1) where .beta..sub.A &gt;.beta..sub.B, V.sub.T '&gt;V.sub.T and output voltage increases; PA1 (2) where .beta..sub.A =.beta..sub.B, V.sub.T '=V.sub.T and output voltage remains unchanged; PA1 (3) where .beta..sub.A &lt;.beta..sub.B, V.sub.T '&lt;V.sub.T and output voltage decreases. PA1 1. In condition R.sub.A &gt;&gt;R.sub.B PA1 .beta.=.beta..sub.A .beta..sub.B, noticing that we have .beta..sub.A &gt;1 and .beta..sub.B &gt;1 for A being N-type and B being P-type material, therefore .beta.&gt;.beta..sub.A or .beta..sub.B. PA1 (i) 1+.gamma..sub.A .DELTA.T=1+.gamma..sub.B .DELTA.T, i.e., .gamma..sub.A =.gamma..sub.B. PA1 (ii).gamma..sub.A .DELTA.T&lt;&lt;1, .gamma..sub.B .DELTA.T&lt;&lt;1 PA1 (i) where .DELTA.T&gt;0 PA1 where .vertline..gamma..sub.A .vertline.&gt;.vertline..gamma..sub.B .vertline., ##EQU22## is more approaching 1 than 1+.gamma..sub.1 .DELTA.T (provided that .gamma..sub.1 =.gamma..sub.A); PA1 where .vertline..gamma..sub.A .vertline.=.vertline..gamma..sub.B .vertline., ##EQU23## is more approaching 1 than 1+.gamma..sub.1 .DELTA.T; where .vertline..gamma..sub.A .vertline.&lt;.vertline..gamma..sub.B .vertline., it is indefinite. PA1 (ii) where .DELTA.T&lt;0, we have: EQU 1+.gamma..sub.A .DELTA.T&gt;1 and 1+.gamma..sub.B .DELTA.T&gt;1 PA1 where .vertline..gamma..sub.A .vertline.&gt;.vertline..gamma..sub.B .vertline., ##EQU24## is more approaching 1 than 1+.gamma..sub.1 .DELTA.T; where .vertline..gamma..sub.A .vertline.=.vertline..gamma..sub.B .vertline., ##EQU25## is more approaching 1 than 1+.gamma..sub.1 .DELTA.T; where .vertline..gamma..sub.A .vertline.&lt;.vertline..gamma..sub.B .vertline., it is indefinite. PA1 where .tau..sub.A &gt;.tau..sub.B, V.sub.T '&lt;V.sub.T, slow response. PA1 where .zeta.&gt;1, i.e., .tau..sub.A &lt;.tau..sub.B, then V.sub.TS &lt;V.sub.T ', quick recovery; PA1 where .zeta.&lt;1, i.e., .tau..sub.A &gt;.tau..sub.B, then V.sub.TS &gt;V.sub.T ', slow recovery.
However, the improvement of sensitivity of gas sensors by the use of such methods is of certain limitation, detections of ppb level can hardly be realized by the sensors manufactured by these methods.
Furthermore, the existing gas sensors are of simple structures constituted by doping a gas sensitive substrate material with characteristic-improving dopant, the selecting characteristic of which is not good enough and the stability is poor. In order to improve the stability in application, a thermosensitive resistor is usually connected externally for compensation, and sometimes, the gas sensitive detecting head is combined with the temperature compensating element. Since the above-mentioned temperature compensating element is a thermosensitive resistor, it can only be used for temperature compensating but not for humidity compensating, without any contribution to the improvement of the selectivity of the gas sensor, and any improving effect to the sensitivity and initial relaxation time of the sensor.